This invention relates to the fabrication of printed circuit boards.
In a typical circuit board processing sequence, copper conductors are formed over the board by electroless deposition of a thin copper layer followed by selective electroplanting of a thick copper layer in the desired conductive pattern utilizing a photoresist. A solder layer is usually applied uniformly over the electroplated copper to act as an etch resist. After removal of the portions of the electroless copper layer and any underlying layer not covered by the electroplated layer, a dry film solder mask is applied over the structure and the board is ready for mounting of semiconductor components. If it is desired to have solder in selected areas, the solder is deposited by screen printing.
In cases where solder is desired on selected portions of the copper conductor, present screen printing techniques are usually adequate. However, certain applications require very dense conductor arrays (i.e., typically less than 625 .mu.m pitch). For such spacing, a more accurate method of providing solder selectively on the conductors is desired.